The invention is related to the deposition of metals on substrates, particularly reinforced polymer laminates of the type used in electronic circuitry.
Electronic circuits are typically formed on reinforced polymer laminates by depositing metals, usually copper, on the surface. The circuit patterns may be made by placing a thin layer of copper foil on the surface of the laminate and then removing portions of the copper to leave the desired pattern. Another method is to deposit the copper from a solution, either as a continuous layer which is treated as just described or preferably, only on areas which form the pattern. Electroplating is not feasible because the laminates are not electrical conductors, hence the method called "electroless plating" has been developed. Solutions of copper salts are prepared and brought into contact with the substrate. In order to deposit the metal from solution it has heretofore been common practice to sensitize the surface of the substrate with a metal which is catalytic to the deposition of copper. Many metals have been suggested to have such capability, but in practice the most commonly used sensitizer has been the combination of palladium and tin salts in an acid solution. An early patent is U.S. Pat. No. 3,011,920 which describes a colloidal solution of such metals. An optically clear, non-colloidal solution is the subject of U.S. Pat. No. 3,682,971.
While metals are currently being used to catalyze the deposition of copper from solution, it will be evident that the use of a separate sensitizing solution involves additional steps and extra costs associated with the metals, particularly the expensive palladium. If it were possible to sensitize the surface of the substrate with less expensive materials, especially by incorporating such materials into the substrate, significant advantages would be obtained in the manufacture of electronic circuits on polymeric laminates and in other applications where electroless deposition of copper is used.